1. Field of the Invention
The invention concerns an optical hydrophone for measurement of the acoustic pressure distribution in a fluid medium, in particular for measurement of an ultrasonic shockwave field or of diagnostic ultrasound.
2. Description of the Prior Art
With acoustic shockwaves as they are used, for example, in lithotripsy, high pressures (up to approximately 108 Pa) occur with rise times of a few ns. The measurement of such high pressures requires sensors with a high mechanical stability. Moreover, these sensors should be miniaturized to a large extent in order to be able to measure the acoustic pressure distribution in a shockwave field with optimally high spatial resolution.
Measurement arrangements in which the light reflected at the free end of a fiber optic cable is used for measurement of the spatial and temporal distribution of the pressure of ultrasound shockwaves in a fluid are respectively known from EP 0 354 229 B1 and DE 38 02 024 A1 as well as from J. Staudenraus, W. Eisenmenger, “Fibre-optic probe hydrophone for ultrasonic and shock-wave measurements in water”, Ultrasonics 1993, Vol. 31, No. 4, page 267-273. This known fiber-optic measurement arrangement makes use of the fact that the high pressure amplitude generates a density change (and thus a change of the index of refraction of the fluid) in the immediate proximity of the free end that modulates the fraction of the light reflected back into the fiber optic cable at the boundary surface. The fiber optic cables used for measurement thereby have a diameter that does not exceed 0.1 mm. The free end of the fiber optic cable that determines the reflectivity of the fluid/fiber optic cable boundary is formed by a spherical or planar end surface standing perpendicular to the fiber optic cable axis. A high spatial resolution, the low directional sensitivity and the high bandwidth that are necessary for the measurement of focused shockwaves are achieved by the minuteness of this end surface.
A fiber-optic shockwave sensor in which the free end of the fiber optic cable is designed as a rotation body whose envelope can be described by a polynomial of the third degree is known from DE 39 32 711 A1. Both the sensitivity and the spatial resolution should be improved via these measures, also given the use of fiber optic cables with a larger diameter.
A fiber-optic hydrophone that uses both changes of the index of refraction of the surrounding fluid and the change of the properties of an interferometer formed at the fiber tip by dielectric layers, in order to increase the sensitivity of the measurement arrangement in this manner, is known from Koch, Ch., “Coated fiber-optic hydrophone for ultrasonic measurement”, Ultrasonics 34, 1996, page 687-689.
A disadvantage of the known fiber-optic hydrophones is that they are very sensitive to breakage and can already be destroyed after 10 to 100 shockwaves at approximately 50 MPa. Moreover, a high production-related expenditure is necessary in order to reproducibly manufacture the free ends of the fiber optic cables with the necessary shape.
Interferometric measurement arrangements in which a polymer film serves as a large-area Fabry-Perot interferometer, that is optically sampled point-by-point such that a two-dimensional image of the acoustic pressure distribution results, are also know in the literature, for example in Beard, P C, Mills T N, “An optical detection system for biomedical photoacoustic imaging”, Proc. SpiE 3916, 2000, page 100-109, or Beard, P C, “Photoacoustic imaging of blood ressel equivalent phantoms”, Proc. SpiE 4618, 2002, page 54-62. Such an apparatus, however, is not suitable for the measurement of shockwaves.